In recent years, a DC/DC converter is widely used in electronic equipment necessitating a stabilized DC power supply. In a step-down DC/DC converter, a primary DC voltage is switched on/off by a switching element, and pulses generated thereby are smoothed by an LC filter to obtain a stabilized voltage. An electric current flowing back during off periods of the switching element is made to flow to a second switching element in place of a diode, to constitute a synchronous DC/DC converter, thereby increasing utilization efficiency of the power supply.
Incidentally, when in the synchronous DC/DC converter the first switching element to implement switching of the primary DC voltage is turned on, the terminal voltage of the second switching element can suddenly increase to increase the voltage of the control electrode because of influence of a parasitic capacitance, thereby leading to false firing (self-turn-on) of the second switching element.
FIG. 12 is a circuit diagram showing a drive circuit of a conventional switching element. The technology used for the aforementioned false firing is, for example as shown in FIG. 12, such that a circuit in which a resistor R9 to decrease the turn-on speed of the first switching element 10a, and a Schottky barrier diode (SBD) D2a to keep the turn-off speed are connected in parallel is connected in series to a control electrode 13a of the first switching element. It is common practice to construct the parallel circuit using a resistance unit consisting of a typical resistor, and an SBD constituted by silicon. It is needless to mention that, for increase in speed of switching, it is desirable to make the wiring length for the circuit connected to the control electrode as short as possible and thereby to decrease inductance.
Furthermore, Patent Literature 1 discloses the technology of preventing the false firing of the second switching element, by generating a negative voltage by a drive circuit of a signal to drive the second switching element and applying the negative voltage to the control electrode during the off periods of the second switching element.